Conventional specimens used to determine fatigue crack growth behavior have the initial crack oriented in such a way that only Mode I displacements are present. The fatigue-crack growth threshold behavior of mild steel in the presence of Mode II displacements was investigated by considering the fatigue behavior of spot-welded joints, where both Modes I and II are present at the point of failure, and by some experiments using specimens designed to give pure Mode II displacements.

It was found that the threshold behavior is controlled by the ease with which a Mode I branch crack forms at the tip of the initial crack. If such a branch forms easily, threshold behavior is controlled by ΔKI for the branch crack, whereas, if branch formation is difficult, threshold behavior is controlled by ΔKI for the initial crack. Branch crack formation seems to be facilitated by the unwanted Mode III displacements which appear when the initial crack tip is curved. For the special case of nominally pure Mode II displacements, failure takes place away from the initial crack front if this is straight, and threshold behavior is determined by other factors.